Displacement-electric signal converter

ABSTRACT

A displacement-electric signal converter having a pair of displacement detecting elements the impedances of which vary in accordance with a displacement, a rectifier including a bridge circuit, first and second capacitors, an oscillator for supplying AC current to the displacement detecting elements, a circuit for controlling voltage for the oscillator, and amplifier for amplifying current flowing through the second capacitor. In this case, the output signal is supplied to a load through a transmission line of a DC electric power source for the amplifier.

United States Patent Saito 1 Aug. 8, 1972 [54] DISPLACEMENT-ELECTRICSIGNAL [56] References Cited CONVERTER UNlTED STATES PATENTS [72]Inventor: Buniiro Saito, Tokyo, Japan 3,456,132 7/1969 Dechelotte..340/l99 [73] Assignee: Kabushikikaisha Yokogawa Denk Seisakusho(Yokogawa Electric Primary Examiner-Thomas B. l-labecker Works,Ltd.), Tokyo, Japan Attorney-Hill, Sherman, Meroni, Gross & Simpson [22]Filed. July 14, 1970 ABSTRACT [21] Appl' 54726 A displacement-electricsignal converter having a pair of displacement detecting elements theimpedances of [30] F i A li ti P i it D m which vary in accordance witha displacement, a rectifier including a bridge circuit, first and secondcapaci- June 23, 1970 Japan ..45/54644 tors an oscillator for supplyingAC current to the placement detecting elements, a circuit forcontrolling [52] US. Cl "340/l 9, 340/210 lt for the oscillator, andamplifier for amplifying [51] Int. Cl. ..G08c 19/08 -em flowing throughthe second capacitor. in this Field of Search 321/13 case, the outputsignal is supplied to a load through a transmission line of a DCelectric power source for the amplifier.

10 Claims, 7 Drawing Figures BACKGROUND OF THE INVENTION the electricsignals to a receiver means disposed at a remote place.

2. Description of the Prior Art A conventional displacement-electricsignal converter has are its power source line and signal line areprovided individually, that is, a four-wire system. Such a conventionalconverter has drawbacks that its cost becomes high, its installation iscomplicated because of the many lines and troubles are apt to occur.

SUMMARY OF THE INVENTION It is a main object of the present invention toprovide a displacement-electric signal converter which is capable oftransmitting an electric signal obtained by con-' verting displacementto a receiver with the so-called two-wire system in which a two-wireline is employed common to power source transmission and signaltransmission.

It is another object of the present invention to provide adisplacement-electric signal converter in which the zero point thereofis not shifted or moved even if its operative range is changed.

It is further object of the present invention to provide a device whichproduces an electric signal in proportion to displacement preciselywithout being affected by surrounding temperature and transmits theelectric signal to a receiver positively and stably.

It is still further object of the present invention to provide a devicewhich can easily attain an intrinsic safety.

Other objects, features and advantages of the present invention will beapparent from the following description taken in conjunction with theaccompanying drawings in which like reference numerals represent likeelements.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an electric connectiondiagram showing one embodiment of the present invention;

FIG. 2 is a schematic diagram illustrating, by way of example, adifferential inductance element employed in the example of the presentinvention depicted in FIG. 1;

FIG. 3 is a graph illustrating characteristics of the ex, ample shown inFIG. 1; and

FIGS. 4 to 7, inclusive, are respectively electric connection diagramsillustrating other embodiments of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an electric wiringdiagram showing one embodiment of the present invention. In the figurereference numeral 1 designates a displacement detecting circuit whichdetects process variables and produces corresponding electrical signals,the process variables being obtained as mechanical displacements.Reference numeral 2 indicates an oscillator, 3 a differential amplifierfor controlling the voltage of an electric power source for theoscillator 2, 4 a range change circuit, 5 an amplifier for amplifyingthe electric signal derived from the displacement detecting circuit l, 6a constant-voltage circuit and 71 and 72 output terminals of thedisplacement-electric signal converter of this invention. Referencenumerals 8 and 9 designate a load .(receiver meter) and a voltage source(an electric power source) which are located at a place remote from themain device consisting of the displacement detecting circuit 1,oscillator 2, differential amplifier 3 and the like. The load 8 and theelectric power source 9 are connected in series between the outputterminals 71 and 72 through transmission lines I. In this case theelectric power source line I is also employed as the signal line I, thatis to say, the transmission system of this example is the so-calledtwo-wire system.

Reference characters L, and L in the displacement detecting circuit 1indicate a pair of coils forming a differential inductance element(inductor). In this case the number of turns of the coils L, and L isselected equal to each other.

FIG. 2 shows, by way of example, a practical embodiment of thedifferential inductance element (inductor). In the figure referencenumeral 11 indicates a core which consists of an E-shaped core member 12and a rod like core member 13 the cross-sectional configuration of whichis approximately rectangular. The coils L, and L are respectively woundon the core member 13. Reference numeral 14 indicates a short-circuitring mounted on the core member 13 between the coils L and L andreference character g a gap formed between the core member 13 and thecenter leg 12C of the core member 12 in which gap g the short-circuitring 14 is disposed. The process variables are produced as mechanicaldisplacements in a well-known conventional manner and thus obtainedmechanical displacements are applied to the short-circuit ring 14in thisexample. When the mechanical displacements are applied to theshort-circuit ring 14, the ring 14 is moved or shifted to the left orright in accordance with the mechanical displacements to change theinductance between the coils L, and L in differential manner.

Turning to FIG. 1, reference character 8 in the displacement detectingcircuit 1 shows a circuit portion which derives the sum and differencebetween currents i, and i passing through the coils L, and L In thecircuit S reference characters D, to D inclusive, indicate diodesrespectively, C, and C capacitors and R, a resistor element. Free endsof the coils L, and L connected in series are respectively connected toinput terminals a and b of a rectifier circuit S, which consists of thediodes D, to D, as a bridge circuit, the connection point of the diodesD, and D corresponding to the input terminal a and the connection pointof the diodes D and D to the input terminal b. The capacitor C, is

inserted between output terminals f and h of the rectifier circuit 8,,namely the connection point of the diodes D D and the connection pointof the diodes D D Reference character T in FIG. 1 designates atransformer a primary winding n, of which is used as an oscillation coilof the oscillator 2. The oscillation output of the oscillator 2 isobtained from a secondary winding n of the transformer T. One end of thesecondary (or output) coil n is connected to the connection point of thecoils L and L while the other end of the coil n is connected to theoutput terminal f of the rectifier circuit S through a capacitor Cdiodes D and D connected in opposite polarities and a filter circuit Sconsisting of the capacitor C and the resistor R connected in parallel.

With the displacement detecting circuit 1 constructed as set forthabove, when the voltage e induced across the output coil n of thetransformer T has the polarity shown in FIG. 1 by symbols 6B ande,current i flows through the coil L,, the diode D the capacitor C and thediode D while current i flows through the coil L the diode D and thediode D namely the currents i and i flow in the coils L and Lrespectively.

When the polarity of the voltage e induced in the coil n is reversed,the current i and i flow through the diode D the capacitor C thecapacitor C the diode D and the coil L and through the diode D thecapacitor C the diode D and the coil L respectively. Consequently,current Ai passing through the capacitor C is the difference between thecurrents i and i passing through the coils L and L a current flowing inthe capacitor C of the filter circuit S is the sum of the currents i andi In this case if the impedances of the diodes, capacitors and resistor(element) are small enough as compared with those of the coils L and Lthe current i and i are respectively expressed as follows:

where (0 represents an angular frequency of the voltage e and L and Limpedance values of the coils L and L2.

Accordingly, the sum of the currents i and i and the differencetherebetween are expressed as follows From the equations (1) and (2) therelationship between i i and L L is deduced as follows With the exampledepicted in FIG. 1 the sum of the currents i and i is delivered from theresistor element R, as the voltage V corresponding to the sum of thecurrents and thus obtained voltage V, is supplied to the differentialamplifier 3 which, in turn, controls the voltage for the oscillator 2 insuch a manner that the sum of the currents i and i may be maintainedsubstantially constant.

References R to R, in the differential amplifier 3 indicate resistorelements respectively which are connected in series with one another. Aconstant voltage from a zener diode D included in the constant-voltagecircuit 6 is applied across the both ends of the series connection ofthe resistor elements R to R,. The connection point between theresistors R and R is connected to a positive input terminal e of thedifferential amplifier 3 while the negative input terminal 9 thereof isconnected to one terminal j of the filter circuit S The other terminal fof the filter circuit 8 is connected to the connection point of theresistors R and R The output voltage of the differential amplifier 3 isapplied to the oscillator 2 as its power source through a diode D whichis used for preventing passage of reverse current, whereby theoscillator 2 starts its oscillation operation. The oscillation output ofthe oscillator 2 is derived from the output coil n of the transformer Tand then applied to the displacement detecting circuit 1 to cause thecurrents i and i to flow in the coils L and L as explained above. Inthis example the power source voltage of the differential amplifier 3 isobtained from the constant-voltage circuit 6 which is supplied withcurrent from the DC power source 9 through the transmission lines I.

The operation of the differential amplifier 3 will be hereinafterexplained. As set forth above the voltage V corresponding to the sum ofthe currents i and i passing through the coils L, and L is applied tothe input terminal 9 of the differential amplifier 3 while the voltage Vproduced across the resistor R is applied to the other input terminal 89of the differential amplifier 3. The differential amplifier 3 producesat its output terminal a voltage corresponding to the difference AE,between the voltages V and V but operates to reduce the differencevoltage AE to substantially zero when the gain thereof is selected highenough. Accordingly, the differential amplifier 3 produces asubstantially constant voltage as its output. The constant voltage ofthe differential amplifier 3 is applied to the oscillator 2 with aresult that the output frequency of the oscillator 2 is substantiallyconstant. Consequently, excitation current applied to the differentialtransformer, namely the sum of the currents flowing through the coils Land L can be controlled to be an approximately constant value. If thesum of the currents i and i is held constant, the difference Ai betweenthe currents i and i which flow through the capacitor C of thedisplacement detecting circuit 1 is exactly proportional to the amountof the displacement of the short-circuit ring 14 mounted on the coremember 13 of the differential transformer as apparent from the equation(3 In the range change circuit 4 shown in FIG. 1, reference characters Rto R inclusive, denote range change resistors, respectively. Theresistance values of the range change resistors R to R are respectivelyselected to have values corresponding to a decimal code such, forexample, as l, 2, 4, 0.1, 0.4 and 0.8. Reference characters S to Sinclusive, indicate switching elements. The resistor R and the switchingelement 8,, and the resistor R and the switching element S 'arerespectively connected in series with each other and the respectiveseries connections of the resistors R to R and the switching elements Sto S are connected to the capacitor C of the displacement detectingcircuit 1 in parallel with one another. As explained in connection withthe displacement detecting circuit 1, as the short-circuit ring 14 ofthe differential inductance element is displaced or shifted to changethe inductance between the coils L and L differentially, the differencecurrent Ai flows through the capacitor C correspondingly. In this caseif one or more of the parallel switching elements S, to S is closed avoltage AV expressed by MR is produced across the resistor or resistorsconnected to the closed switching element. Since the resistors R to Rare selected to follow a decimal code as set forth above, the voltage AVcan be changed with in the range of 0. l-7.0 in steps of 0.1 by suitablyclosing one or more switching elements 8, to S in other words, the rangeof the voltage AV can be variable.

In the constant-voltage circuit 6, reference character 0, indicates atransistor, Q a field-effect transistor and D, a diode which is insertedbetween the base of the transistor Q and one electrode of the Zenerdiode D The other electrode of the Zener diode D is connected to theoutput terminal 72 through a feedback resistor R The source and drainelectrodes of the field-effect transistor Q are connected to the baseand collector electrodes of the transistor Q, respectively and the gateelectrode of the field-effect transistor Q is connected to theconnection point between the diode D and the Zener diode D With thiscircuit construction the gate of the field-effect transistor Q isreverse biased by the forward voltage drop across the diode D so that aconstant current is applied to the diode D As a result, theconstant-voltage circuit 6 operates to keep the power source voltage forthe amplifier substantially constant even if the load current l changes.

In the amplifier circuit 5 reference numeral 51 designates adifferential amplifier and Q a transistor the base electrode of which isconnected to the output terminal of the differential amplifier 51, thecollector electrode of which is connected to the collector electrode ofthe transistor Q and the emitter electrode of which is connected througha resistor R to one end of the feedback resistor R Reference charactersR and R indicate fixed resistors and R a variable resistor. Theseresistors R R and R are connected in series and one end of the resistorR is connected to the emitter electrode of the transistor Q and one endof the resistor R is connected to the output side of the feedbackresistor R The variable resistor R constitutes the zero point adjustingcircuit of this device which will be explained later. The power sourcevoltage for the differential amplifier 51 may be obtained from theconstant-voltage circuit 6 which is supplied with current from the DCpower source 9 through the transmission line I. The input side as of thedifferential amplifier 51 is connected to one end of the movable contactof the variable resistor R while the other input side 9 is connected tothe connection point of the switching elements 8, to S of the rangechange circuit 4.

The amplifier circuit 5, has its 6 input side supplied with the voltageAV produced in the range change circuit 4 while its ea input side issupplied with, as a feedback voltage, the voltage corresponding to thevoltage generated across the feedback resistor R due to the load currentI The transistor 0;, is driven by an output signal from the differentialamplifier 51 to control the load current I,,. The differential amplifier51 operates to make the difference voltage AE, between the voltagesapplied to the input sides 9 and 6 thereof to be substantially zero ifthe gain of the differential amplifier 51 is made high enough. That is,although voltage applied to the input side as of the differentialamplifier 51 is varied in accordance with the variation of the loadcurrent l the differential amplifier 51 operates to bring the voltageapplied to its e input side approximately equal to that applied to its 9input side. In this case since the voltage drops across the resistors Rand R are selected substantially constant, the load current I iscontrolled in such a manner that the variation of the voltage applied tothe as input side of the differential amplifier 51, which is caused bythe voltage drop across the resistor R due to the load current 1 becomesapproximately equal to the voltage AV produced in the range changecircuit 4. The voltage AV can be changed by selectively closing one ormore of the switches S 1 to 8,, so that the load 8 is supplied withcurrent l proportional to the amount of the displacement of theshortcircuit ring 14 and in correspondence with the range through thetransmission line which is also used for power transmission. 7

In a general process control system, a signal transmitted from adetector for detecting process variables to a receiver side is convertedto a uniform or systematic electrical signal of, for example, 4'-*'20milliamperes for the range of 0-100 percent of the input signal of thedetector. In the event that a base current of, for example, 4milliamperes exists in the uniform or systematic electrical signal, whenthe displacement applied to the short-circuit ring 14 of thedifferential inductor is zero, namely the current Ai flowing through thecapacitor C of the displacement detecting circuit 1 is zero, the movablecontact of the variable resistor R is pre viously set or adjusted tomake the load current 1 4 milliamperes. With such an adjustment the zeropoint is prevented from being moved or shifted even if the range isthereafter changed to vary the voltage AV produced in the range changecircuit 4.

FIG. 3 is a graph showing one example of the experimental results withthe device of the present invention shown in FIG. 1 in which theordinate represents the load current in milliampere, the abscissa thedistance X of the short-circuit ring 14 of the differential inductor inmillimeter and the range change resistor R as a parameter. In the graph,a curve a corresponds to the case where R is 5.1 K!) (kilo-ohms); acurve b the case where R 2.4 K9; and curves c and d, the cases where Rare 1 K9 and $00!), respectively.

With conventional apparatus when there is base current, zero pointadjustment must be made every time when the range is changed.

According to the present invention, on the contrary, since the apparatusis adjusted in such a' manner that the load current becomes a basecurrent when an input signal is zero, the zero point is not shifted evenwhen the resistor R is varied as apparent from the graph,

which will means that change of the range can be easily carried out. Inaddition, according to the present invention since an oscillationfrequency of the oscillator 2 may be selected high such, for example, as50 KH the values of the coils L L and the capacitors can be made small.Consequently, the device is safe.

FIG. 4 shows another embodiment of the present invention in whichsimilar references indicate similar elements or components of theexample shown in FIG. 1. In the example shown in FIG. 1 the positiveelectrode or anode of the diode D of the displacement detecting circuit1 is directly connected to the output terminal f of the rectifiercircuit 5,, while in FIG. 4 example the anode of the diode D isconnected to the output terminal f through a resistor R the resistancevalue of which is substantially equal to that of the resistor Rconnected to the cathode of the diode D Further, in FIG. 4 example theinput terminals as and e of the differential amplifier 3 arerespectively supplied with a voltage produced in a resistor R and avoltage across the capacitor C the resistor R being connected betweenthe anode of the diode D and the as input terminal of the differentialamplifier 3, and the capacitor C being inserted between the anode of thediode D and the 6 input terminal of the differential amplifier 3. Thedifferential amplifier 3 of this embodiment thus supplied with thevoltages at its input terminals operates to make the voltage differencebetween the voltages applied thereto to be substantially zero, tothereby control a power source voltage applied to the oscillator 2 sothat it remains approximately constant.

With such an arrangement currents i and i respectively flow through thediodes D and D through the resistors R and R of equal resistance value,namely the currents i and i flow through the rectifier circuit Ssymmetrically, so that an apparatus which is less affected by noises ascompared with the FIG. 1 example is provided.

Although in FIG. 4 the range change circuit 4 is exemplified only by asingle variable resistor R, its detailed construction is substantiallythe same as that shown in FIG. 1.

With this arrangement, it can be also possible as in the case of theFIG. 1 example that the range of the voltage AV may be varied bychanging the value of the resistor R without adjusting the zero pointand also that the zero point may be adjusted by suitably changing thevariable resistor R Other construction, features and operations of FIG.4 example are substantially the same as those of the FIG. 1 example, andthe explanation is omitted for the sake of simplicity.

FIG. 5 shows another modified form of the present invention in whichlike references designate like elements or components to those of theforegoing examples. The main difference in construction between thisexample and the FIG. 4 example resides in the displacement detectingcircuit 1 and the other constructions are substantially same to those ofFIG. 4 example.

In the displacement detecting circuit 1 of the FIG. 5 example, referencecharacter n indicates the secondary coil of the transformer T which isdivided approximately at its center into two coil portions n and n Oneend of the coil, for example, n is connected to the input terminal a ofthe rectifier circuit S and one end of the other coil n to the inputterminal b of the rectifier circuit 5,. The center point of the outputcoil n is connected to the output terminal f of the rectifier circuit Sand one sides of the diodes D and D are connected to the output terminalf through the resistors R and R, which have the same resistance values.In this example the capacitor C is connected to the series circuit ofthe resistors R and R, and in parallel therewith. One end this parallelconnection is connected to the e input terminal of the differentialamplifier 3 and the other end is connected to the 62 input terminal ofthe amplifier 3 through the resistor R According to the example with thedisplacement detecting circuit 1 constructed as set forth above, currentcorresponding to the sum of the inductances of the coils L and L alsoflow through the capacitor C while current proportional to thedifference of inductance flows through the capacitor C The differentialamplifier 3 operates to equalize the voltage across the capacitor C andthe voltage across the resistor R to thereby control the power sourcevoltage to the oscillator 2 with a result that current passing throughthe capacitor C is maintained approximately constant.

The currents i and i flowing through the capacitor C of the FIG. 4example are different in phase by but currents i and i of the FIG. 5example are in phase. Consequently, the capacity values of thecapacitors C and C of this example can be made small as compared withthose of FIG. 1 example and the rectifier circuit S may be manufacturedat a low price.

FIG. 6 is a still another modified form of the present invention inwhich similar reference numerals to those of the foregoing examplesrepresent similar elements or components. In the examples shown in FIGS.1, 4 and 5 the mechanical displacement is converted into correspondingelectric signals by means of inductors, corresponding to inductancechanges thereof but in the FIG. 6 examply the mechanical displacement isconverted into a corresponding electrical signal by the utilization ofcapacitors, corresponding to capacitance changes.

In the FIG. 6 example reference numeral 15 designates a movableelectrode plate one end of which is connected to the output coil n ofthe transformer T and 16 and 17 designate fixed electrode platesadjacent the movable electrode plate 15 but spaced therefrom. The fixedelectrode plates are respectively connected to the output terminals aand b of the rectifier circuit 8,, whereby a differential capacitancetype displacement detector is formed. The other construction andoperation of this example are substantially same to those of theexamples shown in FIGS. 4 and 5.

With such an arrangement when the movable electrode plate 15 isdisplaced in response to the mechanical displacement, the capacitancevalues C and C" between the movable electrode 15 and the fixedelectrodes l6, 17 are varied differentially, whereby current inaccordance with the capacitance variation flows through the rectifiercircuit 8,. In this case the current Ai corresponding to the differencebetween the capacitance values C' and C" flows in the capacitor C,,while the current corresponding to the sum of the capacitance values Cand C flows in the capacitor C as explained in connection with FIG. 1example. The differential amplifier 3 of this example operates toequalize the voltage across the capacitor C with that across theresistor R to thereby control the power source voltage for theoscillator 2 as in the foregoing examples.

FIG. 7 shows a further modified form of the present invention. In thefigure reference numeral 1 designates the displacement detecting circuitwhich includes coils L and L 2 the oscillator, 3 a control circuit,corresponding to the differential amplifier 3 in the foregoing examples,to control the-power source voltage for the oscillator 2 and 5 thecurrent amplifier circuit which employs a blocking oscillator.

The oscillator 2 of this example is a back coupling type oscillatoremploying a transistor 0,. This oscillator 2 oscillates at the naturalfrequency of a tuning circuit consisting of a feedback winding n f ofthe transformer T connected to the base of the transistor Q and acapacitor C connected in parallel to the winding mf. The oscillationamplitude of the oscillator 2 is changed in accordance with themagnitude of the power source voltage applied thereto. The oscillator 2is supplied with a voltage from a power source including the Zener diodeD which is, in turn, supplied with a current from the DC power source 9through the transmission line 1. The control circuit 3 comprises atransistor and is inserted between the oscillator 2 and the Zener diodeD The base of the transistor Q is connected to the connection point of aresistor R and a Zener diode D through a resistor R the seriesconnection of the resistor R and the Zener diode D being connected tothe Zener diode D in parallel, whereby the base of the transistor Q issupplied with a constant reference or standard current I The inductancevalues of the coils L, and L in the displacement detecting circuit 1 isdifferentially changed in accordance with the displacement of theshort-circuit ring 14 as described above, so that the capacitor Cpermits therethrough passage of the current corresponding to thedifference between the inductance values of the coils L and L while thecapacitor C permits therethrough passage of the current corresponding tothe sum of the inductances of the coils L and L In this example theoutput terminal j of the rectifier circuit S is connected to the base ofthe transistor Q of the control circuit 3 to apply the currentcorresponding to the sum of the inductances of the coils L and L to thetransistor Q in the counterdirection to the reference current I; asshown by a dotted line in the figure. The current amplifier circuit 5includes a blocking oscillator 51 which has a transistor Q and coils n fand n To the input side of the blocking oscillator 51 is connected theaforementioned filter circuit 8,. The power source of the blockingoscillator 51 is obtained from the Zener diode D which is supplied withcurrent from the DC power source 9 through the transmission line I asdescribed above, while a bias current I is applied from the Zener diodeD through a resistor R In the figure reference numeral 52 is a circuitincluding transistors Q and O, which rectifies and amplifies an outputof the blocking oscillator 51 produced at the secondary winding n of atransformer T to control the load current I flowing through thetransmission line 1. Reference character R indicates a variable resistorwhich is connected between the output end of the feedback resistor R andthe output side of the filter circuit S and feedbacks current I, due toa voltage produced in the resistor R I0 by the load current to the inputside of the blocking oscillator 51.

With the arrangement shown in FIG. 7 the current corresponding to thesum of the inductances of the coils L and L; are compared with thereference current I from the Zener diode D and the difference currenttherebetween is applied to the base of the transistor Q so that as thesum current is increased the base current of the transistor 0 isdecreased with a result that the voltage between the collector andemitter of the transistor Q, is increased to reduce the power sourcevoltage to the oscillator 2, while as the sum current is decreased toenhance the base current of the transistor Q, the voltage across thecollectoremitter of the transistor 0,, is lowered with a result that thevoltage to the oscillator 2 is enhanced and the power source voltage forthe oscillator 2 is controlled to maintain the sum current approximatelyconstant. The current passing through the capacitor C, which correspondsto the difference between the inductances of the coils L and L is fed tothe blocking oscillator 51 of the current amplifier circuit 5, wherebythe voltage across the capacitor C of the blocking oscillator 51 isincreased. As a result, the'bias current I flows as a base current ofthe transistor Q so that the transistor Q; is made conductive byblocking oscillation thereof. When the transistor O is made conductiveand its output is applied to the transistor Q through the sametransformer T the current as to the emitter current of the transistor Qflows in its collector through resistors R. and R The pulse voltagecaused thereby is smoothed by a capacitor C connected to the resistor Rand then supplied to the base of the transistor Q, which, in turn,controls the load current I flowing through the transmission line I inaccordance with the voltage supplied to its base. The load current 1causes generation of a voltage across the resistor R which produces thefeedback current I, flowing through the resistor R As a result, theinput current is amplified such that the input current is balanced withthe feedback current I,. Consequently, the output current I flowingthrough the load 8 corresponds to the difference between the inductancesof the coils L and L Since in the present example the currentcorresponding to the difference between the inductances of the coils Land L is amplified by the current amplifier circuit employing theblocking oscillator as set forth above, the present example is lessaffected by variations of temperature of the surroundings.

It is possible in FIG. 7 example to employ capacitors shown the FIG. 6instead of the coils L and L A change of the range is obtained bycontrolling variable resistor R It will be apparent that manymodifications and variations may be effected without departing from thescope of the novel concepts of this invention.

We claim as our invention:

1. A displacement-electric signal converter comprismg:

a pair of displacement sensing means connected in series and havingimpedances which are differentially changed in accordance with amechanical displacement;

a rectifier circuit consisting of a plurality of diodes connected as abridge circuit, said series-connected pair of displacement sensing meansbeing connected between the input terminals of said bridge circuit,first and second capacitors, said first capacitor being connectedbetween the output terminals of said bridge circuit, said secondcapacitor having means connecting one end to the connection point ofsaid pair of displacement sensing means and its other end connected toone of the output terminals of said bridge circuit;

a current corresponding to the difference between the impedances of saidpair of displacement sensing means being obtained from said firstcapacitor;

a current corresponding to the sum of said impedances being obtainedfrom said second capacitor;

an oscillator with its output side coupled to said pair of displacementsensing means for applying A. C. current to said pair of displacementsensing means; differential amplifier circuit with one input terminalconnected to said second capacitor, the other input terminal of saiddifferential amplifier supplied with a constant voltage and the outputside of said differential amplifier connected to said oscillator forproviding the electric power source voltage for said oscillator tomaintain substantially constant current flowing through said secondcapacitor;

a second amplifier to which the voltage across both terminals of saidfirst capacitor is supplied for amplifying current flowing through saidfirst capacitor; and

means including a D. C. electric power source of said second amplifierfor transmitting the output signal of said second amplifier to a load.

2. A displacement-electric signal converter as claimed in claim 1 inwhich said pair of displacement sensing means consist of a pair ofdifferential inductors.

3. A displacement-electric signal converter as claimed in claim 1 inwhich said pair of displacement sensing means consist of a pair ofcapacitors.

4. A displacement-electric signal converter as claimed in claim 1 inwhich said second amplifier is a second differential amplifier one inputterminal of which is supplied with voltage across said first capacitorand the other input terminal of which is supplied with a feedbackvoltage, said second difierential amplifier controlling a load currentto make the voltage difference between said voltages applied to itsinput terminals substantially zero.

5. A displacement-electric signal converter as claimed in claim 1 whichcomprises a circuit for comparing current passing through said secondcapacitor with a reference current and controlling the electric powersource voltage of said oscillator in accordance with the differencebetween said currents, and a third amplifier employing a blockingoscillator for amplifying current passing through said first capacitor.

6. A displacement-electric signal converter as claimed in claim 1 whichfurther includes a range change circuit connected in parallel to saidfirst capacitor for changing the range of voltage across said firstcapacitor.

7. A displacement-electric signal converter as fllllill ssait'sraatrasrc a ar a ras sistor and a switching element connected inseries, and each of said circuits connected in parallel with said firstcapacitor.

8. A displacement-electric signal converter as claimed in claim 4 whichincludes a zero point adjusting circuit which adjusts the zero point ofthe converter by controlling the feedback voltage from said seconddifferential amplifier.

9. A displacement-electric signal converter as claimed in claim 8 inwhich said zero point adjusting circuit consists of a variable resistorelement.

10. A displacement-electric signal converter as claimed in claim 5 inwhich a means is provided for changing the range by varying a feedbackcurrent of said blocking oscillator.

1. A displacement-electric signal converter comprising: a pair of displacement sensing means connected in series and having impedances which are differentially changed in accordance with a mechanical displacement; a rectifier circuit consisting of a plurality of diodes connected as a bridge circuit, said series-connected pair of displacement sensing means being connected between the input terminals of said bridge circuit, first and second capacitors, said first capacitor being connected between the output terminals of said bridge circuit, said second capacitor having means connecting one end to the connection point of said pair of displacement sensing means and its other end connected to one of the output terminals of said bridge circuit; a current corresponding to the difference between the impedances of said pair of displacement sensing means being obtained from said first capacitor; a current corresponding to the sum of said impedances being obtained from said second capacitor; an oscillator with its output side coupled to said pair of displacement sensing means for applying A. C. current to said pair of displacement sensing means; a differential amplifier circuit with one input terminal connected to said second capacitor, the other input terminal of said differential amplifier supplied with a constant voltage and the output side of said differential amplifier connected to said oscillator for providing the electric power source voltage for said oscillator to maintain substantially constant current flowing through said second capacitor; a second amplifier to which the voltage across both terminals of said first capacitor is supplied for amplifying current flowing through said first capacitor; and a means including a D. C. electric power source of said second amplifier for transmitting the output signal of said second amplifier to a load.
 2. A displacement-electric signal converter as claimed in claim 1 in which said pair of displacement sensing means consist of a pair of differential inductors.
 3. A displacement-electric signal converter as claimed in claim 1 in which said pair of displacement sensing means consist of a pair of capacitors.
 4. A displacement-electric signal converter as claimed in claim 1 in which said second amplifier is a second differential amplifier one input terminal of which is supplied with voltage across said first capacitor and the other input terminal of which is supplied with a feedback voltage, said second differential amplifier controlling a load current to make the voltage difference between said voltages applied to its input terminals substantially zero.
 5. A displacement-electric signal converter as claimed in claim 1 which comprises a circuit for comparing current passing through said second capacitor with a reference current and controlling the electric power source voltage of said oscillator in accordance with the difference between said currents, and a third amplifier employing a blocking oscillator for amplifying current passing through said first capacitor.
 6. A displacement-electric signal converter as claimed in claim 1 which further includes a range change circuit connected in parallel to said first capacitor for changing the range of voltage across said first capacitor.
 7. A displacement-electric signal converter as claimed in claim 6 in which said range change circuit includes a plurality of circuits each comprising a resistor and a switching element connected in series, and each of said circuits connected in parallel with said first capacitor.
 8. A displacement-electric signal converter as claimed in claim 4 which includes a zero point adjusting circuit which adjusts the zero point of the converter by controlling the feedback voltage from said second differential amplifier.
 9. A displacement-electric signal converter as claimed in claim 8 in which said zero point adjusting circuit consists of a variable resistor element.
 10. A displacement-electric signal converter as claimed in claim 5 in which a means is provided for changing the range by varying a feedback current of said blocking oscillator. 